Without knowing how you are going to be building your script, this example gives some suggestions on how to access the values (there might be better ways which you can find potentially find from the builtin scripts)
Names Default To Here(1);
path = "$BUILTIN_SCRIPTS/Power_Two_Independent_Sample_Means.jsl";
Include(path);
show(win << Get Window Title); // take a look at Power_Two_Independent_Sample_Means.jsl and you see this reference is there
// Using indexing from Show Properties
// Window("Power Explorer for Two Independent Sample Means")["Power Explorer for Two Independent Sample Means","Explorer Settings",NumberEditBox(1)]
aneb1 = win[Outline Box("Explorer Settings"), NumberEditBox(1)];
// Using references
// _alphanb gets lost somewhere, so access through alphatb
alpha_neb2 = (alphatb << sib);
// Using XPath
alpha_neb = win << XPath("//TextBox[text() ='Alpha']/following-sibling::NumberEditBox");
alpha_neb3 = alpha_neb[1]; // XPath returns a list
// Setting the values
alpha_neb2 << Set(0.10, run script(1)); // You most likely want to include Run Script(1) to most value changes to avoid messing anything up
-Jarmo